Drive assembly for acoustic sources

ABSTRACT

Drive assembly for acoustic sources having sound emitting surfaces adapted to be excited into vibration movements, in particular for use in seismic explorations. The assembly comprises an electric rotational motor (17) with an associated axle which comprises an axle part (18) the outer cross-sectional contour of which at least partially in the axial extension of the axle part is non-circular, but preferably smoothly rounded. Further the assembly comprises a number of push rods (21, 22) being arranged radially in relation to the axle (18) and adapted to preferably indirectly at their radially inner ends (23, 24) to be influenced by the non-circular axle part (18) during rotation of the axle, whereas the radially outer ends of the push rods (21, 22) are adapted to excite said sound emitting surfaces (1) into vibrational movement.

This invention relates to a drive assembly for acoustic sources havingsound emitting surfaces adapted to be excited into vibrational movement,in particular for use in seismic prospecting.

Sources employed for generating sound waves in water can for example besonar sources, flextensional sources or seismic transmitters or energysources. Advantageously the invention can be employed for such types ofsources, i.e. for emitting sound waves under water. Upon reflection fromthe sea bed and underlying geological formations resulting echo signalscan be detected by means of hydrophones or geophones of various types.

It is well known that low frequency sound waves can be transmitted overlonger distances through water and geological structures than highfrequency sound waves can. Within military applications as well aswithin the marine sector of oil and gas industry there has for a longtime been a need for powerful low frequency sound sources which canoperate under water. Sources of various constructions and designs forthese purposes and fields of use, have been available for a long time.Such acoustic sources are for example described in Seismic EnergySources 1968 Handbook, Bendix, United Geophysical Corporation 1968, andin Transducer Needs for Low-Frequency Sonar, Proceedings of the SecondInternational Workshop on Power Transducers for Sonic and Ultrasonics,France, June 12-13, 1990.

Most of the acoustic sources employed today are of the impulsive type,in which efforts are made to have the sources emit as much energy aspossible during as short time as possible. The frequency contents ofsuch a source can be modified only to a very small degree, and differentsources are selected for different surveying problems.

In recent time there have been developed seismic energy sources in theform of vibrators which can vibrate within various frequency bands,so-called "frequency sweep". To this group there belong vibrators whichoperate by employing hydraulic means and sources employing piezoelectricor magnetostrictive materials. In hydraulic vibrators a piston iscontrolled by a valve arrangement, and thereby it is possible to obtainlarge oscillation amplitudes. The piezo-electrical effect as knowninvolves a change of length of a crystalline material when an electricalvoltage is applied to the outer surfaces thereof, and conversely that anelectrical voltage is generated when the material is subjected to aphysical deformation. Magnetostriction means that a magnetic materialbeing subjected to a magnetic field change will undergo a length change,and conversely that an applied length change of the material will giverise to a change of the magnetic field.

There are various different manners of designing acoustic sources. Forlow frequency uses it is common to let the sources have a circularsurface (in the form of a piston) when the hydraulic principle isemployed, and a cylindrical shape with either a circular or ellipticcross-section when piezoelectric and magnetostrictive materials areused.

A concept where a hydraulic piston source is employed, is described inThe Marine Vibrator Source, First Break Vol. 6 No. 9, September1988/285.

The greatest problem with this type of controllable sources is to obtaina well defined and sufficiently high amplitude of the oscillations. Inorder to obtain this there will be a need for either a large sourcesurface or a small source surface having high oscillation amplitudes.

Vibrators based on the hydraulic principle (for example within marineseismic exploration) provide high amplitudes at low frequencies. Thepiston movements are controlled by a valve arrangement. The degree ofcontrol of these hydraulic piston sources as regards amplitude combinedwith frequency, is limited, however.

The availability of so-called high-magnetostrictive magnetic materialshas improved the possibilities of manufacturing good acoustic sources.By using this type of materials as drive elements it is possible toobtain amplitude changes which can be up to 20 times larger than thecorresponding amplitudes in the case of a piezoelectric material.Sources employing high-magnetostrictive materials have been commerciallyavailable for several years. The amplitude improvement, however, isstill relatively modest, and the magnetostrictive materials have seriousrestrictions at low frequencies, even though the control of theamplitude is simple and very exact.

A normal design of the actual driver shall be discussed closer here,taking as a starting point a cylindrical source having an ellipticcross-section: The cylindrical casing consists of an elastic membrane orshell. Inside and in parallel to the cylinder axis and in engagementagainst the shell, there are provided two end beams. The cross-sectionsurface of the beams is a symmetrical mirror image in relation to theshort axis of the elliptic shell, and each beam is delimited by thatportion of the shell which faces the end of the long axis and a cordparallel to the short axis. Between the beams and in engagement withtheir parallel side faces there is provided an electrically controlleddrive element in the form of a drive rod. The longitudinal axis of thedrive rod coincides with the long axis of the elliptically shapedcross-section and is equally spaced from the end faces of the source. Ifthe magnetostrictive principle is employed, the drive rod consists of amagnetic and preferably a high-magnetostrictive material, which ismagnetized by the surrounding electric coil according to the desiredfrequency of the source. If the piezoelectric principle is used thedrive rod is made of a piezoelectric material. Of course the drive rodcan consist completely or partially of any material which provides forthe desired possibility of length changes.

The fundamental design of an acoustic source as described above can varyin details. An acoustic source having a cylindrical shape and anelliptical cross-section having drive rods of high-magnetostrictivematerial is described inter alia in U.S. Pat. No. 4,901,293 "A rareearth flextensional transducer".

The invention relates to a basic concept for what is referred to aboveas a drive element, but which in the following description will bereferred to as a drive assembly or drive pack. As to the remaining partsof a source a starting point is taken in a design having a cylindricalshape and an elliptical cross-section, as mentioned above. The sourcehas an elastic membrane or shell and two inner end beams at the ends ofthe long axis.

The drive system in its broadest sense comprises according to anembodiment of the invention, a frame having an outer shape as aright-angled parallelepiped. A drive assembly consisting of an electricmotor coupled to an axle having an oval and conical shape is attached tothe frame. The oval and conical shaped axle is in engagement with anadjustable cradle. Against the cradle there are provided push rods whichtransfer the force to the membrane. The electric motor is connected to amotor regulator which controls the frequency. By rotation of the ovaland conically shaped axle, the membrane is caused to oscillate backforth in proportion to the rotational velocity. Depending upon how theoval and conical axle is shaped, large low frequency amplitudes can begenerated.

The amplitude is varied in this embodiment by displacing the adjustablecradle axially in relation to the oval and conical axle, by using alinear motor. In this method movement amplitude acting on the push rodsbeing connected to the membrane through the end beams, can be varied. Byvarying the frequency with a motor regulator and the movement amplitudewith the linear motor, it is possible to obtain selectable frequencysweeps having the desired amplitude and frequency.

In this embodiment the height of the frame is adapted so as tosubstantially correspond to the distance between the parallel innersides of the end beams, i.e. the length of the long axis between the endbeams. The width of the frame corresponds to the axial length of thecylindrical shell. The thickness of the frame can be varied without muchrestriction, and can for example be made of rectangular beams, T beams,L beams, I beams, tubes or other structural elements of suitablematerials. As described below the elliptically shaped end plates of thesource are attached to the height-thickness sides of the frame.

A desired mechanical bias is provided for by devices on the push rods.The push rods, which transfer the forces to the membrane, are providedwith a screw device for example, which makes it possible to obtainlonger or shorter rods, which means that the mechanical bias can bevaried.

The concept described here for designing an acoustic source involves alarge degree of freedom as regards structure, dimensions and acousticpower, since among other things a number of drive assemblies in a drivesystem can be freely selected. The frame involves good possibilities formounting the assemblies or drive packs and for attaching the end beamsduring manufacture of the membrane.

In view of the background given above and as will appear from thefollowing description, the drive pack or assembly according to theinvention in its most general form is characterized by an electricrotational motor having an associated axle which comprises at least oneaxle part the outer cross-sectional contour of which at least partiallyin the axial extension of the axle part, is non-circular, but preferablysmoothly rounded, for example in the form of an oval cross-sectionalcontour, and a number of push rods being arranged radially in relationto the axle and preferably indirectly at their radially inner ends,adapted to be influenced by the non-circular axle part during rotationof the axle, whereas the radially outer ends of the push rods areadapted to cause vibrational movement of said sound emitting surfaces.

The invention as well as additional specific features thereof will beexplained more closely in the following description, with reference tothe drawings.

FIG. 1 shows a cylindrical acoustic source having an ellipticalcross-section, including a drive pack according to the invention

FIG. 2 shows details comprised by the drive pack.

FIG. 3 shows glide wheels being mounted in the cradle.

FIGS. 4A-4F illustrate various cross-sectional shapes of thenon-circular axle part, and

FIG. 5 shows an alternative embodiment to what is illustrated in FIGS. 2and 3.

An acoustic source in which the drive system according to the inventionpreferably can be employed, is shown in FIG. 1. As will be seen thesource has a cylindrical shape with an elliptical cross-section.Externally the source consists 5 of a casing surface in the form of anelastic membrane 1 and end plates 2 and 3. Inside the membrane there aretwo end beams 4 and 5 at the ends of the long axis of the ellipticalcross-section.

The drive pack is mounted within a rectangular frame 6. The frame islocated centrally inside the membrane in such a way that a plane midwaybetween and parallel to both the height-width sides coincides with theplane of all long axes. The height h of the frame is so adapted that itgenerally corresponds to the free space between the end beams, which inpractice means that the frame will engage the end beams. The width b ofthe frame corresponds to the axial length of the cylindrical membrane.Therefore the end plates of the the source can be attached with screws7, 8, 9 and 10 to the height-width sides of the frame. The thickness tof the frame, i.e. the spacing between the two height-width sides, isbroadely determined by the requirements to a practical building-in ofthe drive pack as well as the requirements as to dimensions ofthrough-openings for the push rods. Since the frame is "floating" withinthe source when this--by means of the push rods--is arranged in amechanically biassed condition, the frame as such will not be subjectedto any significant mechanical strain. Accordingly, the height and thewidth of the frame will mainly be determined by the number of push rodsor drive packs being needed in order to obtain the desired acousticpower. Through-openings 11, 12, 13 and 14 for the push rods of the drivepack, are located in the frame along a common central axis or line.Through-openings 15, 16 for the motor axle and for the linear motor areprovided in the frame normally to the openings for the push rods.

An example of a drive assembly or pack according to the invention isshown in FIG. 2. An electric motor 17 is coupled to an oval andconically shaped axle 18. Against this axle there is engaged adisplaceable cradle 19, 20 which acts against the pair of push rods 21,22. At the end of the push rods there are shown glide wheels 23, 24mounted for reducing the friction against the movable cradle 19, 20. Inthe movable cradle there are provided glide wheels 25, 25 for reducingthe friction against the oval and conically shaped axle. Also otherfriction reducing means can be contemplated, where the actual bearingcan be associated with the oval and conically shaped axle. By means of athreaded extension piece 27, 28 the push rods can be rotated and therebythe mechanical bias can be changed. The method of the mechanicallybiassing the push rods can take various forms, as for example by usingwedges or hydraulic pistons.

The RPM of the electric motor 17 can be regulated by means of a controlsystem 29 which is connected to the electric motor through an electriccable 31. This control system can comprise for example a frequencyconverter or the like, depending upon whether an AC motor or a DC motoris used. The amplitude is varied by means of an axle having an oval andconical shape 18 and by a movable cradle 19, 20 engaging the axle andbeing adapted to be axially displaced along the axle by means of alinear motor 30. Instead of a linear motor various other devices andmeans can be employed for causing a sufficient movement of the cradle.Another way of controlling the amplitude may be to have the position ofthe eccentric axle radially blocked by means of a key and can bedisplaced axially in relation to the axle of the motor by using forexample a linear motor, as explained below with reference to FIG. 5.

In FIG. 3 an example is shown of how a glide wheel 25 is mounted on anaxle 32 in the cradle in order to reduce the friction against the ovaland conically shaped axle.

The basic concept described with respect to the source as far as detailsare concerned, can be designed in a number of different manners, thesebeing comprised by the invention. In addition to the choice of thenumber of push rods, amplitude generating axles, drive packs and framedimensions as already mentioned above, this can be the location of themotor 17 outside the actual source as represented by the membrane 1 andas indicated schematically in FIG. 2.

The amplitude generating axle previously described as the oval andconically shaped axle 18, can be designed in various manners whereby thedecisive parameter will be the desired frequency content. For example adesign of the amplitude generating axle may be contemplated, forproviding a number of frequencies simultaneously without changing themotor frequency. A few such modifications of the cross-sectional contourof the axle are discussed below with reference to FIG. 4. Moreover, itis fully possible that the source is designed so as to be used for onespecific RPM and/or that the amplitude generating axle is replaceablefor different uses. In certain uses it may be of interest only to have acylindrical and not a conical amplitude generating axle.

If it is desired to extend the high frequency range of the source, driveelements based upon magnetostrictive or piezoelectric materials can beintegrated in the push rods, as shown at 28A and 28B in FIG. 2.

Evidently, the drive pack can also be employed in sources of cylindricalshape and having a cross-section other than elliptical, for example acircular cross-section. Piston sources obviously represent another useof the drive pack, where for example a piston and a membrane of circularshape can be used, or two diametrically opposed pistons with circularmembrane shapes, being arranged against each other. The membrane ofcourse can have a different geometry from the circular shape.

FIGS. 4A-4F illustrate various shapes of the axle or more closelydefined the axle part, as indicated on the preceding page. In FIGS.4A-4F the actual motor axle is designated 38, whereas the cross-sectionand the contour of the specifically shaped axle parts for obtainingvarious frequency contents, is shown completely black around the actualdrive axle 38.

The cross-sectional shape in FIG. 4F corresponds to the oval contourwhich has been discussed above with reference to FIG. 2. FIG. 4A shows apossibly still simpler elementary shape based upon a circular contour,which, however, due to its eccentricity in relation to the axle 38,represents a form of non-circularity which results in a fundamentalfrequency equal to half the basic frequency of the embodiment in FIG.4F, provided that the rotational velocity of the drive axle 38 is thesame.

FIG. 4B shows a modification in relation to the oval cross-section inFIG. 4F, wherein four projecting contour elements 34B1, 34B2, 34B3 and34B4 give the excitation through the push rods a content of overharmonicfrequencies with respect to the basic or fundamental frequency.

FIG. 4C shows a more edged cross-sectional contour with eight triangularprojections or waves, which in contrast to the other examples shown donot have any significant rounding of the outer cross-sectional contour.

On the other hand FIG. 4D shows a well rounded cross-sectional contourhaving four wave crests. Finally FIG. 4E shows an embodiment of the axlepart having a circular basic form modified by two groups consisting eachof three wave crests, namely 34E1, 34E2, 34E3 and 34E4, 34E5, 34E6respectively.

The variants of cross-sectional contours illustrated in FIGS. 4A-F showsome of the possibilities being available as regards excitation with aspecifically desired frequency content.

As also mentioned on the preceding page the amplitude regulation cantake place by means of an axially displaceable axle part on the drive ormotor axle. This embodiment is in principle shown in FIG. 5, where thedrive axle 48 of the motor carries an axle part 58 constituting aseparate part being axially displaceable on the drive axle 48. In allaxially mutual positions, however, the axle part 58 is rotationallylocked to the drive axle 48 by means of a key 48A thereon and acorresponding keyway 58A internally in the axle part 58. Purelyschematically in FIG. 5 there is shown how two push rods 51 and 52 withtheir inner ends directly engage the outer surface or contour of axlepart 58, so as to be actuated thereby during rotation. Thus, insimilarity to the embodiment in FIG. 2, also the drive axle 48 and pushrods 51, 52 always assume the same mutual position in axial direction,whereas displacement of the axle part 58 on the drive axle 48 can servefor amplitude control, which is based on the precondition that thenon-circular axle part or member 58 is conical.

Finally, FIG. 5 shows that on the axle 48 in addition to unit 50 therecan be provided an additional unit 60 of the same construction, i.e. anaxle part 68 (or several such parts) with an associated set of push rods61, 62 for excitation of the same sound emitting surfaces (not shown) inthe acoustic source. Two or more such units 50, 60 can also be arrangedon separate axles for each unit, these axles being then preferably allparallel.

I claim:
 1. Drive assembly for acoustic sources having vibrationallyexcitable sound emitting surfaces, said drive assembly comprising:anelectric rotational motor having an axle which comprises at least oneaxle part, wherein the circumference of at least one transversecross-section of the axle part is non-circular, wherein the axle parthas a conicity, and a push rod having an inner end and an outer end,wherein the push rod is arranged radially in relation to the axle,wherein the inner end of the push rod is communicable with the axlepart, wherein the sound emitting surfaces are excitable into vibrationalmovement by the outer end of the push rod.
 2. Drive assembly accordingto claim 1, further comprising an axially displaceable, cradle-likedevice for transferring the rotational movement of the axle part intotranslational movement of the push rod.
 3. Drive assembly according toclaim 2, wherein said cradle-like device comprises an electric linearmotor for axial displacement of the cradle-like device.
 4. Driveassembly according to claim 1, further comprising a control system forregulating the rotations per minute (RPM) of the rotational motor. 5.Drive assembly according to claim 1, wherein said push rod comprises aplurality of push rods, each having an inner end and an outer end,wherein each push rod is arranged radially in relation to the axle. 6.Drive assembly according to claim 2, wherein said cradle device (19, 20)comprises glide wheels for running on said axle part having axlessubstantially parallel to the axle of the rotational motor.
 7. Driveassembly according to claim 1, wherein the non-circular axle part has across-sectional shape with one or more projecting contour elementsadapted to excite said sound emitting surfaces through the push rod at adesired frequency.
 8. Drive assembly according to claim 1, wherein thepush rod comprises a biasing device for adjusting the length of the pushrod.
 9. Drive assembly according to claim 1, wherein the push rodcomprises a drive element for a supplementary or superposed excitationof said sound emitting surfaces by means of the push rod.
 10. Driveassembly according to claim 1, wherein said drive assembly is locatedsubstantially within and in general surrounded by the associatedacoustic source, and wherein the rotational motor is located outside theacoustic source.
 11. Drive assembly according to claim 1, furthercomprising a plurality of parallel axles wherein each axle of saidplurality of parallel axles comprises one or more non-circular axleparts wherein each axle part has a cooperating set of push rods. 12.Drive assembly according to claim claim 1, wherein the axle part isdisplaceable along the longitudinal axis of the axle and rotationallylocked to the axle, wherein the push rod is position so that the innerend has the ability to be influenced by the axle part during rotation ofthe axle.
 13. Drive assembly according to claim 9, wherein said drivedement of said push rod comprises a magnetostrictive element.
 14. Driveassembly according to claim 9, wherein said drive element of said pushrod comprises a piezoelectrical element.
 15. Drive assembly for acousticsources having vibrationally excitable sound emitting surfaces, saiddrive assembly comprising:an electric rotational motor having an axlewhich comprises at least one axle part, wherein the circumference of atleast one transverse cross-section of the axle part is non-circular, anda push rod having an inner end and an outer end, wherein the push rod isarranged radially in relation to the axle, wherein the inner end of thepush rod is communicable with the axle part, wherein the sound emittingsurfaces are excitable into vibrational movement by the outer end of thepush rod, wherein the push rod comprises a biasing device for adjustingthe length of the push rod.
 16. Drive assembly according to claim 15,wherein the push rod comprises a drive element for a supplementary orsuperposed excitation of said sound emitting surfaces by means of thepush rod.
 17. Drive assembly according to claim 16, wherein said driveelement of said push rods comprises a magnetostrictive element. 18.Drive assembly according to claim 16, wherein said drive element of saidpush rods comprises a piezoelectrical element.
 19. Drive assemblyaccording to claim 15, wherein said push rod comprises a plurality ofpush rods, each having an inner end and an outer end, wherein each pushrod is arranged radially in relation to the axle, wherein each push rodcomprises a biasing device for adjusting the length of each push rod.